The human eye is a very delicate, complex, and imperfect optical system. Over much of the last 250 years, techniques for measuring and correcting the optical impairments of the eye have been limited to addressing nearsightedness or farsightedness, and the corresponding cylindrical refractive errors. The human eye, however, also shows higher order refractive errors, or so-called “higher aberrations”. As the light levels decrease, the quality of a person's vision is affected more by the higher aberrations than by cylindrical refractive errors. For example, pupils dilate in twilight situations in order to project more light onto the retina of the eye. As a result, light rays pass through the peripheral regions in the eye where greater refractive errors are present. Therefore, even a person with normal 20/20 vision has a decreased visual acuity under critical illumination conditions. By accessing and correcting the higher order refractive errors, visual performance can be significantly improved.
Wavefront analysis is a developing technology which has been shown to significantly enhance operational aspects of refractive surgery on the human eye. In particular, as the eye focuses on an image, flat sheets of light (wavefronts) passing through the eye are distorted by the imperfect refractive medium. Hence, a wavefront will tend to distort on it's way through an irregular cornea or lens. Simple refractive errors, like nearsightedness or farsightedness, normally result in a simple bowl-like, symmetrical wavefront distortion. Higher order aberrations, however, can yield a more complex, non-symmetrical distortion of the originally flat wavefront, which is unique for every person's eye. These wavefront distortions will then lead to blurred optical imaging of viewed scenes.
Recent advances in integrated wavefront sensing technology allow for the measurement of simple refractive errors, as well as for the measurement of higher order aberrations. These measurements are now performed with previously unknown precision and speed. Thus, the refractive power of an individual's eye can be spatially measured over the diameter of the pupil, and based on the measured individual wavefront distortions, a person's visual acuity can be improved. When eyeglasses are inadequate to make the necessary improvements, refractive surgery may be necessary. These improvements can be accomplished in one of several ways.
One approach to vision correction by refractive surgery involves the external pre-compensation of errors in the wavefront using adaptive optics. By reflecting the wavefront of a viewed scene at a deformable active mirror in the adaptive optics, a distortion can be introduced which inversely matches the wavefront distortion of the eye. The wavefront distortion of the eye and the active mirror then cancel each other, and the patient sees a perfectly sharp image without higher order refractive errors. Based on this phenomenon, reliable micro-mechanical active mirrors can be used in a closed-loop system, where the measured distortions are directly converted into surface changes of the mirror. It happens that these distortions can be used for refractive surgery.
With refractive surgery, the corneal tissue is ablated using a focused laser beam. More specifically, this treatment can be based on individually measured wavefront aberrations, with tissue ablation permanently neutralizing the refractive errors of the patient's eye. A system for accomplishing tissue ablation is disclosed in U.S. Pat. No. 6,610,051, titled “A Device and Method for Performing Refractive Surgery”, issued to Bille and assigned to the same assignee as the present application, i.e. 20/10 Perfect Vision Optische Geraete GmbH. Further, it is well known to those skilled in the art that accurate and precise refractive surgery requires the corneal tissue be photoablated when the eye is substantially stabilized or stationary. It is also well known that the eye is naturally stabilized following a saccadic movement of the eye.
Saccadic movements of the eye are the rapid, ballistic like movements of the eye used in scanning an observed scene. These movements are involuntary, and occur even when the eye is apparently fixed on a given object or fixation point. It is possible, however, to initiate a saccadic movement of the eye at a predetermined moment in time by moving a fixation point through an arc of about 5° (five degrees). After each such saccadic eye movement, there is a latency period of approximately 0.12 seconds when the eye is substantially stabilized.
For many reasons, it is desirable to perform photoablation of the corneal tissue when the eye is stabilized. Such stabilization is best assured if photoablation is accomplished during the latency period that follows a saccadic eye movement. It may be critical, therefore, to coordinate the laser procedure with the saccadic movement of the eye. It happens, however, that stabilization of the eye through only saccadic eye movement may not be adequate, in many instances, to allow for laser cutting. This is so because there are other physiological phenomena that may cause the eye to move. For example, the beating of a patient's heart, as well as the inhaling and exhaling associated with a respiration cycle, causes the eye to move. Obviously, either of these movements can prevent the accurate photoablation of the corneal tissue. Importantly, both of these physiological events are rhythmic in nature, and both include a period of non-activity.
In light of the above, it is an object of the present invention to provide a system for spatially stabilizing a selected base point on the optical axis of a patient's eye during the latency period following a saccadic eye movement. Another object of the present invention is to provide a system stabilizing the base point, following a saccadic movement of the eye, during the resting period in a heartbeat sequence, and during the relaxation period in a respiration cycle of the patient. Yet another object of the present invention is to provide a system for photoablating corneal tissue during the period of time when the base point, and hence the eye, is substantially stabilized. Still another object of the present invention is to provide a system for stabilizing a base point on the optical axis of the eye that is easy to use, relatively simple to manufacture, and comparatively cost effective.